Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $21.51   6.625%   $22.935   $22.94   $22.95   $22.90  
  Purchase #2 $202.85   3.198%   $209.3371   $209.34   $209.35   $209.30  
  Purchase #3 $217.93   11.390%   $242.7522   $242.75   $242.75   $242.80  
  Purchase #4 $5.51   4.666%   $5.7671   $5.77   $5.75   $5.80  
  Purchase #5 $12.87   5.791%   $13.6153   $13.62   $13.60   $13.60  
  Purchase #6 $22.90   3.128%   $23.6163   $23.62   $23.60   $23.60  
  Purchase #7 $131.85   8.390%   $142.9122   $142.91   $142.90   $142.90  
  7 purchase totals = $615.42       $660.9352   $660.95   $660.90   $660.90